x86 - add speculation_barrier pattern

This patch adds a speculation barrier for x86, based on my understanding of the required mitigation for that CPU, which is to use an lfence instruction.

This patch needs some review by an x86 expert and if adjustments are needed, I'd appreciate it if they could be picked up by the port maintainer. This is supposed to serve as an example of how to deploy the new __builtin_speculation_safe_value() intrinsic on this architecture.